Polarization diverse phase dispersionless broadband antenna

ABSTRACT

A multi-octave bandwidth antenna is disclosed which radiates with variable polarizations using: a metal ground plane, first and second blade antenna elements, first and second coaxial transmission line feeds, and a 180° hybrid coupler. The two blade antenna elements are fixed above the metal ground plane in proximity with each other, and fed respectively by the central conductors of the first and second coaxial transmission line feeds. The 180° hybrid coupler has a sum and difference port to control the polarization of waveforms of the antenna.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment of anyroyalty thereon.

BACKGROUND OF THE INVENTION

The present invention relates generally to broadband antennas, and morespecifically to a multi-octave bandwidth antenna which will radiate andreceive electromagnetic energy at UHF-Band, L-Band, C-Band, S-Band,X-Band, K-Band and beyond with variable radiated polarization.

The task of providing an antenna element which will radiate and receiveover multi-octave bandwidths is alleviated, to some extent, by thefollowing U.S. Patents, which are incorporated herein by reference:

U.S. Pat. No. 3,680,127 issued to D. J. Richard on Jul. 25, 1972;

U.S. Pat. No. 3,015,101 issued to E. Turner et al on Dec. 26, 1961;

U.S. Pat. No. 3,509,465 issued to Andre et al on Apr. 28, 1970; and

U.S. Pat. No. 3,618,104 issued to L. Behr on Nov. 2, 1971.

U.S. Pat. No. 3,618,104 discloses a broadband low-profile circularlypolarized antenna having a form factor comprising a cornucopia-shapedelement. U.S. Pat. No. 3,509,465 discloses a tunnel diode amplifierintegrated into a printed circuit equiangular spiral antenna in whichthe antenna elements are used as a portion of the amplifier transmissionline.

U.S. Pat. No. 3,680,127 discloses a tunable omni-directional antennahaving two loaded, concentric, semicircular radiating members. U.S. Pat.No. 3,015,101 discloses an antenna consisting of one or more elementseach essentially a coplanar equiangular stub antenna with a folded overshorted base, the general configuration being that of a scimitar blade.

While the systems described above are exemplary in the art, the needremains to provide a multi-octave antenna element which has excellenttime dispersion properties; and will radiate and receive at UHF-Band,L-Band, C-Band, S-Band, X-Band, K-Band and beyond, and radiates withvariable polarization by shifting the phase of the input radio frequency(RF) signal. The present invention is intended to satisfy that need.

SUMMARY OF THE INVENTION

The present invention comprises a broadband antenna which will radiateand receive electromagnetic energy at UHF-Band, L-Band, C-Band, S-Band,X-Band, K-Band and beyond. In addition, this antenna has excellent timedispersion properties over a broadband of operation. The presentinvention consists of two radiating elements above a ground plane. Thecurvature of each of the radiating elements from the mouth to a pointnear the tip thereof is an arc of constant radius which results in a lowvoltage standing wave ratio. The antenna is fed with two coaxialtransmission lines connected to the ground plane and to the antennaelements.

Variation in the polarization orientation is obtained by feeding the RFsignals to the transmission lines using a 180° hybrid coupler which hasa sum port and a difference port. When the RF is applied to the sumport, the elements are fed in phase and a vertically polarized waveformresults. When RF is applied to the difference port, the elements are fedin anti-phase (180° out of phase) and a horizontal polarized waveformresults.

When the same RF signal is applied to the sum and difference port, a 45°linearly polarized waveform is radiated. If one input is shifted by 90°,a circularly polarized waveform is radiated.

It is an object of the present invention to provide a broadband antennawhich transmits and receives multi-octave electromagnetic energy.

It is another object of the present invention to radiate and receive atUHF-Band, L-Band, C-Band, S-Band, X-Band, K-Band and beyond.

It is another object of the present invention to radiate at variablepolarizations including vertically polarized waveforms, horizontallypolarized waveforms, and circularly polarized waveforms.

These objects together with other objects, features and advantages ofthe invention will become more readily apparent from the followingdetailed description when taken in conjunction with the accompanyingdrawings wherein like elements are given like reference numeralsthroughout.

DESCRIPTION OF THE DRAWINGS

FIG. 1a is a side view of the antenna of the present invention;

FIG. 1b is a plan view of the antenna of FIG. 1A;

FIG. 2a is an illustration of the broadband antenna of the presentinvention and its image elements;

FIG. 2b illustrates the dual antenna;

FIG. 3 illustrates the operation of the antenna of the present inventionas a transmission line slot in a metal ground plane;

FIG. 4 is a facsimile of the antenna surge impedance through thetransmission line and antenna;

FIG. 5 details the feeding of the present invention;

FIGS. 6-8 illustrate the effects of phase shift on input radio frequencysignals on the polarization of waveforms produced by the presentinvention;

FIG. 9 illustrates three different geometries of the blade elementsorientation in the antenna.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is a multi-octave bandwidth antenna which willradiate and receive electromagnetic energy at UHF-Band, L-Band, C-Band,S-Band, X-Band, K-Band and beyond with variable radiated polarizations

Two developments in the state-of-the-art which led to the presentinvention are disclosed in two patent applications of Michael C. Wicksand Paul Van Etten. The first is entitled "The Mono-Balde PhaseDispersionless Antenna" filed on Mar. 5, 1986, Ser. No. 06/841,376, nowabandoned the disclosure of which is incorporated by reference. Thisantenna is composed of a metal ground plane with a Mono-Balde Antennaelement fixed above it and achieves multi-decade bandwidth.

The second development is entitled "The Bi-Blade Century BandwidthAntenna" filed on Mar. 5, 1986, Ser. No. 06/841,381, the disclosure ofwhich is specifically incorporated by reference. This antenna achievescentury bandwidth (i.e., 100) using a pair of blades fixed in proximityof each other and a coaxial transmission line feed.

The Polarization Diverse Phase Dispersionless Broadband Antenna,described in detail below, has several important properties which makeit superior to available broadband antennas. Among these properties are:(1) the antenna has a large bandwidth; (2) the antenna has little or nophase dispersion; (3) the input VSWR (Voltage Standing Wave Ratio) ofthe antenna is extremely good (less than 1.2 to 1); (4) the antenna ispolarization diverse, i.e., horizontally polarized, or verticallypolarized, or circularly polarized, or elliptically polarized, andpolarization diversity is obtained using only a single variable phaseshifter; (5) the antenna is a nonresonant structure, which contributesto its broadband nature; (6) the antenna is ideally suited to beingemployed in a phased array, providing a large bandwidth, high gain, andgood directivity; (7) the antenna is simple to construct and inexpensiveto manufacture as compared to other broadband antennas.

The Polarization Diverse Phase Dispersionless Broadband Antenna isconstructed with two radiating elements above a metal ground plane. Thereader's attention is now directed towards FIGS. 1a and 1b which aremechanical schematics of an embodiment of the antenna of the presentinvention. FIG. 1a is a side view of the antenna; and FIG. 1b is a sideview of the antenna of FIG. 1a. To understand the theory of operation ofthis antenna, the reader's attention is directed towards FIG. 2a, whichillustrates: the broadband antenna of the present invention, and itsimage elements which are below the ground plane.

With FIG. 2a in mind, Image Theory is applied to the antenna of FIG. 1by constructing a dual antenna. The construction of a dual antenna isaccomplished by removing the ground plane 100 from the antenna of FIG.2a. The result is the dual antenna of FIG. 2b.

The pair of radiating elements XX' (XY) in FIG. 2b can be considered anangled, tapered, balanced transmission line slot in a metal groundplane, (See FIG. 3). The slot width increases approximatelylogarithmically from the throat (Point A) to the mouth (Point B) of theantenna. The slot transmission line has a transverse electromagneticmode of propagation. To understand the radiation mechanism of thetapered balanced slot transmission line, consider the pair of radiatingelements, which are fed with a 50 ohm coaxial line. The width and heightof the slot at Point A is designed such that the surge impedance atPoint A is exactly 50 ohms. The surge impedance is measured through thecoaxial transmission line into the feed point of the slot transmissionline at Point A, progressing through Point B, through Point C, ontoPoint D, (See FIG. 3).

FIG. 3 illustrates the operation of the antenna of the present inventionas a transmission line slot in a metal ground plane. For a detaileddescription of conventional slot antennas, see Chapters 8 and 9 of"Antenna Engineering Handbook" by Henry Jasik and published by theMcGraw Hill Book Company in 1961, the disclosure of which isincorporated by reference.

FIG. 4 is a chart of the antenna surge impedance through thetransmission line and antenna of the present invention. The surgeimpedance is measured with a Time Domain Reflectometer of similarapparatus. A desired Time Domain Reflectometer display of the surgeimpedance is seen in FIG. 4. The curvature of the radiating elementgoing from Point B (the antenna mouth) to Point C is approximately anarc of constant radius. If the radius is too small, the slope of thecurve in FIG. 4 from Point A to Point B will be excessive and provideunwanted reflections back to the input, thus causing a large input VSWR.If the radius is made too large, the physical size of the elementbecomes excessive, making the antenna large and bulky. The geometry ofthe element from Point C to Point D is an arc of constant radius. Thedesign compromise which results in the configuration seen in FIG. 1provides an overall tradeoff between antenna geometry, physical size,and VSWR. The physical shape of the radiating element from Point D toPoint E to Point F to Point G is relatively unimportant and is astraight line for manufacturing ease. The distance from Point G to PointH is made at least ten times the amount of the slot opening at Point A,the antenna throat, (See FIG. 3). This provides a containment offringing of the electric field lines, while also providing mechanicalrigidity.

The manner in which the Polarization Diverse Phase DispersionlessBroadband Antenna is fed with coaxial transmission line is describedwith the aid of FIG. 5. The outer conductor of the coaxial transmissionline is secured (possibly soldered) to the ground plane. The centerconductor of the coaxial transmission line is attached to the radiatingelement at Point H. The exact position is determined while inspectingthe surge impedance with a Time Domain Reflectometer, such that the"surge impedance bump" is trimmed out. Both radiating elements are fedin this fashion.

The two coaxial feed cables are connected to the output ports of abroadband magic tee (180 degree hybrid coupler). When an RF signal isapplied to the sum port of the magic tee, the two radiating elements arefed in phase, the electric field extends from the radiating element tothe ground plane, and a vertically polarized waveform is radiated, (SeeFIG. 6).

When an RF signal is applied to the difference port, the two radiatingelements are fed in antiphase, (180 degrees out of phase), and ahorizontally polarized waveform is radiated, (See FIG. 7). When the samesignal is applied to the sum port and the difference port of the magictee, a 45 degree linearly polarized waveform is radiated. If one of theinputs in FIG. 6 or FIG. 7 is phase shifted by 90 degrees with respectto the other, a circularly polarized waveform is radiated.

The fact any polarized waveform can be radiated from the PolarizationDiverse Phase Dispersionless Broadband Antenna by employing only onephase shifter is another very important feature. For the reception orradiation of electromagnetic energy, this antenna system saves aconsiderable amount of money on construction and maintenance cost overother available broadband antennas. The simplified feed structure forpolarization diversity provides an additional cost savings. The antennaconfiguration including this unique feed is illustrated in FIG. 8.Different polarizations are obtained as the amount of the phase shift isvaried, and some different values of phase shift and the correspondingpolarization are listed in FIG. 8. The variable phase shifter is theonly additional component required to obtain all differentpolarizations, unlike conventional antennas, which require variableattenuators as well.

The manner in which the radiating elements are supported can varyaccording to the particular application. The elements should be mountedsuch that no metal be placed near the regions of Point A, or Point B, orPoint C, or Point D in FIG. 3. The support structure is generally foundto work well when the elements are supported anywhere along the straightedge between Point E and Point F. The antenna in FIG. 1 is structurallysuperior to the dual antenna shown in FIG. 2b, and can easily beemployed in a phased array configuration. Also, the coaxial transmissionline is the ideal feed for the Polarization Diverse Phase DispersionlessBroadband Antenna due to its geometry and because the coaxialtransmission line is phase dispersionless.

Three different geometries of the Polarization Diverse PhaseDispersionless Antenna are illustrated in FIG. 9. The antenna in FIG. 9ais described above. The antennas in FIGS. 9b and 9c operate in a fashionsimilar to the antenna in FIG. 9a, but the different geometries lendthemselves to applications where different structural requirementsexist. The antenna in FIG. 9b is composed of a slot transmission lineabove a ground plane for radiating vertical polarized energy, and adiverging plate transmission line for radiating horizontal polarizedenergy. The geometry and position of the radiating elements is chosen toforce the surge impedance to increase linearly from the throat to themouth of the antenna for both the slot transmission line (for verticalpolarization) and the diverging plate transmission line (for horizontalpolarization). The antenna configuration in FIG. 9c is the same as inFIG. 9b, except that the slot transmission line and the diverging platetransmission line are interchanged for the vertical and horizontalpolarization respectively.

For a typical example, the Polarization Diverse Phase DispersionlessAntenna with dimensions:

Element Length: 23 inches;

Mouth Opening; 12 inches;

Element Thickness: 0.1 inches; has the measured performance of:

Frequency: 8 GHz;

Gain: 19.1 dB;

Vertical Beamwidth: 19 degrees;

Horizontal Beamwidth: 17 degrees;

VSWR: 1.16 to 1;

Polarization Isolation: 35.2 dB.

From March through May 1985 three experimental antennas were built: oneeach corresponding to those of FIGS. 9a, 9b, and 9c. Measurements madeat the Verona Test Site with a Time Domain Reflectometer gave excellentresults, indicating both a low VSWR (Voltage Standing Wave Ratio) andthe linear surge impedance versus distance characteristics seen in FIG.4. The two prototype antennas in FIGS. 9a and 9c are being tested,obtaining spatial patterns and polarization responses at UHF-Band,L-Band, S-Band, C-Band, X-Band, and K-Band, to demonstrate the extremelybroadband performance of these antennas. These bands are presented belowin Table 1.

                  TABLE 1                                                         ______________________________________                                        Frequency Band Frequency                                                      ______________________________________                                        UHF              300-1,000 Mc                                                 L              1,000-2,000 Mc                                                 S              2,000-4,000 Mc                                                 C              4,000-8,000 Mc                                                 X               8,000-12,500 Mc                                               K                18-26.5 Gc                                                   ______________________________________                                    

The Polarization Diverse Phase Dispersionless Broadband Antenna willradiate and receive electromagnetic energy at UHF-Band, L-Band, S-Band,C-Band, X-Band, K-Band and beyond. Furthermore, the antenna hasexcellent time dispersion properties (i.e., dispersionless) over anextremely broadband of operation. The antenna is polarization diverse,i.e., horizontally polarized, or vertically polarized, or circularlypolarized, or elliptically polarized. Also, polarization diversity isobtained using only a single variable phase shifter, making dualpolarization easy to implement in new radar systems. This antenna designlends itself to stacking or arraying to form a large phased array formany different applications.

While the invention has been described in its presently preferredembodiment it is understood that the words which have been used arewords of description rather than words of limitation and that changeswithin the purview of the appended claims may be made without departingfrom the scope and spirit of the invention in its broader aspects.

What is claimed is:
 1. An antenna comprising:a metal ground plane; firstand second blade antenna elements fixed in proximity with each otherabove said metal ground plane; first and second coaxial transmissionline feeds each having a central conductor respectively connected to thefirst and second blade antenna elements; and a means of feeding signalswith controlled phase into said first and second coaxial transmissionline feeds.
 2. An antenna, as defined in claim 1, wherein said first andsecond blade antenna elements each comprise:a blade element which has athroat, a mouth, and a tip, said throat serving as a feed point andbeing electrically connected to a central conductor of one of said firstand second transmission line feeds, said throat being comparativelynarrow compared to other portions of the blade element, said mouth beinga mid-section of said blade element and also the blade element's widestsection, said tip being an arc of constant radius, said tip therebyresulting in a low voltage standing wave ratio of about 1.119 to
 1. 3.An antenna, as defined in claim 2, wherein said means of feeding signalscomprises a 180° hybrid coupler which is electrically connected withsaid coaxial transmission line feeds, said 180° hybrid coupler having asum and difference port such that when a single radio frequency signalis fed into the sum port, the first and second blade antenna elementsare fed in phase and a vertically polarized waveform results, when asingle radio frequency signal is applied to the difference port, thefirst and second blade elements are fed in antiphase (180° out-of-phase)and a horizontally polarized waveform results, and when a same radiofrequency signal is applied to both the sum and difference port, a 45°linearly polarized waveform results.
 4. An antenna, as defined in claim3, wherein said antenna includes an array of first and second bladeantenna elements fixed in proximity with each other above said metalground plane, and fed with a plurality of first and second coaxialtransmission line feeds.
 5. An antenna, as defined in claim 4, whereinsaid first and second blade antenna elements each has a length of abouttwenty two inches, a mouth width of about seven inches, and a width ofabout 0.1 inches.